The rotational pulling process known as the Czochralski (CZ) method is representative of the methods for manufacturing silicon single crystals used to prepare silicon wafers. In the production of a silicon single crystal by the CZ method, as is commonly known, a seed crystal is immersed in a silicon melt that has been prepared in a quartz crucible, then is pulled upward while the crucible and the seed crystal are rotated. Thereby, a silicon single crystal is grown downward from the seed crystal.
In a silicon single crystal produced in this way, various grown-in defects are known to be formed which cause problems during device fabrication. Grown-in defects are generally of two types: dislocation clusters which arise in an interstitial rich region, and crystal originated particles (COPs) or voids which arise in a vacancy rich region. Between these two regions, there exists a ring-like oxygen-induced stacking fault (OSF) forming region. In addition, there are vacancy type and interstitial type grown-in defect-free regions. The typical defect distribution in the radial direction of the single crystal is described below while referring to FIG. 1.
A ring-like OSF-forming region exists at an intermediate position in the radial direction of the crystal. Inside the ring-like OSF-forming region, a COP or void-forming region is located, while intervening a defect-free region therebetween. Outside the ring-like OSF-forming region, a dislocation cluster-forming region is located, while intervening an oxygen precipitation accelerating region and an oxygen precipitation inhibiting region (Pi region) therebetween. The oxygen precipitation accelerating region is a vacancy type grown-in defect-free region which is a region free of vacancy type grown-in defect (PV region), and the oxygen precipitation inhibiting region is an interstitial type grown-in defect-free region which is a region free of interstitial type grown-in defects (PI region).
Such a defect distribution is known to be controlled by two factors. One is the crystal pull rate, and the other is the temperature distribution within the crystal just after solidification. The influence of the crystal pull rate is described below while referring to FIG. 2.
FIG. 2 shows the defect distribution in a longitudinal section view of a single crystal grown while gradually lowering the pull rate. At a rapid pull rate, the ring-like OSF-forming region is positioned at the outside edge of the crystal. Therefore, in wafers obtained from a single crystal grown at a high pull rate, COPs are formed substantially throughout the crystal in the radial direction thereof. As the pull rate decreases, the ring-like OSF forming region moves gradually toward the center of the crystal, where it eventually vanishes. Hence, in wafers obtained from a single crystal grown at a low pull rate, dislocation clusters are formed substantially throughout the entire crystal in the radial direction thereof. Incidentally, the cross-section of the crystal shown in FIG. 1 corresponds to the sectional view taken at position A in FIG. 2.
Both dislocation clusters and COPs are harmful grown-in defects that have deleterious effects on device characteristics. However, because COPs are less harmful and given the demand for high productivity, up until now growth has been carried out entirely under high pull rate conditions such that the OSF-forming region as shown in the region above position D in FIG. 2 is either to be positioned at the outside edge of the crystal or to be excluded entirely from the crystal.
However, with the rapid trend in integrated circuits towards smaller geometries in recent years, even the harmfulness of COPs has become of concern. Hence, there has arisen a need to prevent the formation of not only dislocation clusters, but also COPs. Prior-art which addresses this need includes the growth of defect-free crystals using point defect distribution control in the manner described in Patent Reference 1 and Patent Reference 2.
In Patent References 1 and 2, the above-mentioned effect to control the defect distribution by adjusting the temperature distribution within the crystal just after solidification is utilized for achieving the grown-in defect-free crystals.
That is, in conventional CZ pulling, just after the crystal solidifies, the crystal releases heat from its outside surface. Hence, with regard to the axial temperature gradients within the crystal just after solidification, the temperature gradient Ge at the circumferential portion of the crystal tends to be larger than the temperature gradient Gc at the center portion. As a result, in the defect distribution, especially the ring-like OSF-forming region, as seen in a longitudinal section view of a single crystal grown while gradually lowering the pull rate, the center portion is projecting downwardly and the projecting portion is pointed V-like shape. Hence, even when pull conditions near the critical pull rate at which the ring-like OSF-forming region vanishes at the center of the crystal are used, a grown-in defect-free region are formed to a limited extent only at the center of the crystal, and it is not possible in this way to obtain the defect-free region throughout the crystal in the radial direction thereof.
Here, dislocation clusters and COPs do not, of course, arise in the defect-free region inside the ring-like OSF-forming region. Nor do they arise in the ring-like OSF-forming region itself or in the oxygen precipitation accelerating region or the oxygen precipitation inhibiting region outside of the ring-like OSF-forming region. That is, these four regions are all grown-in defect-free regions.
By modifying the hot zone structure in the crystal pulling furnace so as to thermally insulate the crystal just after solidification from the outside surface thereof, the temperature gradient Gc at the center portion of the crystal can be made the same as or larger than the temperature gradient Ge at the circumferential portion of the crystal. In this way, as shown in FIG. 3, in the longitudinal section view of a single crystal grown while gradually lowering the pull rate, the shape of the ring-like OSF-forming region acquires a U-like shape which retains the downwardly projecting tendency but the center portion is flattish. Moreover, together with using the modified hot zone structure, by employing pull conditions near the critical pull rate at which the ring-like OSF-forming region disappears at the center of the crystal, the crystal can be made free of defects throughout the entire crystal in the radial direction thereof. Here, in FIG. 3, these pull rate conditions fall within the B-C range.
Other related art for achieving a defect-free crystal involves pulling the crystal in a hydrogen atmosphere, as described in Patent Reference 3 and Patent Reference 4. Such an approach in which a very small amount of hydrogen gas is mixed into the inert gas introduced into the pulling furnace, can inhibit the formation of vacancy defects in the same way as nitrogen doping of the silicon melt.
In a process in which a grown-in defect-free crystal is grown by controlling the defect distribution as described in Patent Reference 1 and Patent Reference 2, a low pull rate near the critical pull rate at which the OSF-forming region vanishes at the center of the crystal must be selected as the crystal pulling condition. Hence, a decrease in productivity is unavoidable.
Moreover, the range of the pull rate for achieving a grown-in defect-free crystal (margin: the B-C range in FIG. 3) is narrow, making it difficult to stably grow a grown-in defect-free crystal. It is thus difficult to achieve a grown-in defect-free crystal over the entire length of the crystal, resulting in a lower production yield of grown-in defect-free crystal. For this reason, reducing the production costs of grown-in defect-free crystals has been a challenge. In particular, as the crystal diameter has increased to 200 mm and 300 mm, it has become increasingly difficult to satisfy the relationship Ge<Gc and the range B-C of the pull rate for achieving the defect-free crystal has tended to become even narrower. Accordingly, there exists a desire for new technology capable of overcoming this hurdle.
With regard to the range of the pull rate for obtaining a grown-in defect-free single crystal, because the prior-art range of the pull rate for producing a grown-in defect-free crystal (margin: the range E-C in FIG. 3) is narrow, in a prior-art grown-in defect-free crystal that has been pulled, the oxygen precipitation-accelerating region which is a vacancy type grown-in defect-free region (PV region) and the oxygen precipitation-inhibiting region which is an interstitial type grown-in defect-free region (PI region) (and also the ring-like OSF region in the case of a low-oxygen crystal containing 12×1017 atoms/cm3 or fewer) end up intermingled. As a result, there is a possibility that oxygen precipitation properties such as the oxygen precipitate density and size, as well as the denuded zone (DZ) width, may not be uniform in the in-plane direction of the wafer.
That is, because the PV region and the PI region are intermingled within the wafer, the oxygen precipitate distribution in a device process will be non-uniform. Therefore, both areas having a strong gettering ability and areas having a weak gettering ability exist. Also, an active region in the vicinity of a surface layer of a device must be free not only of COPs and dislocation clusters, but also of oxygen precipitates and secondary defects thereof such as OSFs and punch-out dislocations. Yet, the width of the region in which such defects are absent, i.e., the DZ width, is non-uniform in the in-plane direction of the wafer. These non-uniform distributions in the intrinsic gettering (IG) ability and DZ width result in a variability in the device characteristics and a lower yield. To avoid such non-uniformities, it would be desirable to have the ability to produce a grown-in defect-free crystal composed only of a PV region or a PI region. However, even were it possible to produce a grown-in defect-free crystal composed only of a PV region, because of the ready tendency of oxygen to precipitate, it would be necessary to prevent forming oxygen precipitates and secondary defects thereof within the active region of the device. Therefore, the allowable oxygen concentration range would have to be limited to a low-oxygen region (e.g., [Oi]<12×1017 atoms/cm3), which makes it impossible to manufacture in a high-oxygen region.
Therefore, it had been hoped that grown-in defect-free crystals composed only of a PI region which does not allow oxygen precipitates and their secondary defects to form in the active region of the device even in a high-oxygen region could be stably and efficiently grown; however, the margin width of the pull rate for obtaining a grown-in defect-free crystal composed entirely of a PI region has hitherto been very narrow, which makes it impossible to prepare wafers composed entirely of a PI region.
(Patent Reference 1) Japanese Patent Application, First Publication No. 2001-220289
(Patent Reference 2) Japanese Patent Application, First Publication No. 2002-187794
(Patent Reference 3) Japanese Patent Application, First Publication No. 2000-281491
(Patent Reference 4) Japanese Patent Application, First Publication No. 2001-335396